Characterization of Nosema ceranae Genetic Variants from Different Geographic Origins

In recent years, large-scale colony losses of honey bees (Apis mellifera) have been reported and the infection with the microsporidia Nosema ceranae has been involved. However, the effect of N. ceranae at the colony level and its role in colony losses vary in different geographic areas. This difference may be related to the presence of multiple N. ceranae genetic variants resulting in different biological consequences. In this study, we analyzed the genetic diversity of 75 N. ceranae samples obtained from 13 countries and Hawaii through inter-sequence single repetition (ISSR) and evaluated if two of these genetic variants triggered different immune responses when infecting Apis mellifera iberiensis. The genetic diversity analysis showed that 41% of the samples had the same DNA amplification pattern, including samples from most European countries except Spain, while the remaining samples showed high variability. Infection assays were performed to analyze the infection levels and the immune response of bees infected with N. ceranae from Spain and Uruguay. The infected bees presented similar infection levels, and both isolates downregulated the expression of abaecin, confirming the ability of the microsporidia to depress the immune response. Only N. ceranae from Uruguay downregulated the expression level of imd compared to control bees. On the other hand, both genetic variants triggered different expression levels of lysozyme. As imd and lysozyme play important roles in the response to pathogens, these results could reflect differences in the biological consequences of N. ceranae variants in A. mellifera infection.